Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/08—Mirrors
  • G02B5/0816—Multilayer mirrors, i.e. having two or more reflecting layers
  • G02B5/085—Multilayer mirrors, i.e. having two or more reflecting layers at least one of the reflecting layers comprising metal
  • G02B5/0875—Multilayer mirrors, i.e. having two or more reflecting layers at least one of the reflecting layers comprising metal the reflecting layers comprising two or more metallic layers
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
  • G02B1/10—Optical coatings produced by application to, or surface treatment of, optical elements
  • G02B1/11—Anti-reflection coatings
  • G02B1/113—Anti-reflection coatings using inorganic layer materials only
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
  • G02B1/10—Optical coatings produced by application to, or surface treatment of, optical elements
  • G02B1/11—Anti-reflection coatings
  • G02B1/113—Anti-reflection coatings using inorganic layer materials only
  • G02B1/115—Multilayers
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/08—Mirrors
  • G02B5/0883—Mirrors with a refractive index gradient
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/20—Filters
  • G02B5/28—Interference filters
  • G02B5/281—Interference filters designed for the infrared light
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/20—Filters
  • G02B5/28—Interference filters
  • G02B5/285—Interference filters comprising deposited thin solid films
  • G02B5/288—Interference filters comprising deposited thin solid films comprising at least one thin film resonant cavity, e.g. in bandpass filters

Definitions

• the present invention relates to optical-thin-film structures and methods for making them.
• an “optical thin film” is a layer of material(s) having a thickness that is on the order of the coherence length of the light with which it interacts. Such optical thin films typically have a thickness within the range of a fraction of a nanometer to many micrometers.
• optical-thin-film structures comprising plural optical thin films, and having appropriate characteristics, are useful for optical applications.
• a few examples of such optical-thin-film structures include mirrors, wavelength filters, anti-reflection coatings, and windows.
• Optical-thin-film structures often comprise multiple layers of dielectric materials, typically alternating between those comprising materials having a relatively high refractive index and those having a relatively low refractive index.
• IR infrared
• dielectric materials typically alternating between those comprising materials having a relatively high refractive index and those having a relatively low refractive index.
• low-index materials/layers are those having a refractive index less than about 1.7
• high-index materials/layers are those having a refractive index of about 1.7 and greater.
• optical thin films composing such structures should be robust to temperature changes, mechanical shock, humidity, and other environmental factors. Moreover, the optical thin films of optical reflectors, filters, and windows must absorb no more than a minimal amount of light over a broad range of wavelengths.
• Si Silicon
• Germanium Ge
• ZnSe ZnS
• ZnS ZnS
• Thorium Fluoride (ThF 4 ,) was the preferred low-index material, but its radioactivity has limited its practical uses.
• Some other fluoride compounds that have low refractive index and exhibit relatively low absorption losses in the IR spectrum e.g., AIF 3 , BaF 2 , SrF 2 , NaF, LiF, MgF 2 and CaF 2 , etc. have been used as the low-index layer for certain optical applications.
• the present invention provides optical-thin-film structures consisting essentially of fluorides as a low-index material, and methods for making such structures.
• at least one low-index thin film consists essentially of a major amount (/.e., > 50 weight percent) of Barium Fluoride (BaF 2 ) and a minor amount of one or more secondary fluoride compounds.
• the secondary fluoride compounds suitable for use in conjunction with embodiments of the invention include those that: (1) reduce the average crystal size of a low-index optical thin film (relative to that of a low-index optical thin film consisting solely of Barium Fluoride), (2) have suitable optical properties for the particular optical application (e.g., refractive index, absorption losses, etc.), and (3) have suitable chemical stability.
• Fluoride compounds that satisfy requirements (1) through (3) above for a variety of optical applications include, without limitation : CaF 2 , AIF 3 , YF 3 , YbF 3 , CeF 4 , ThF 4 , NaF, LiF, KF, SrF 2 , and MgF 2 .
• the amount of the secondary fluoride compound which may include one or more suitable fluoride compounds— is typically in the range of about 1 to about 25 weight percent, as a function of: (1) the particular secondary fluoride compound used, (2) substrate temperature and the temperature of the deposited thin films, and (3) required film thickness, among any other parameters. More typically, the amount of the secondary fluoride will be in the range of about 1 to about 15 weight percent.
• Optical-thin-film structures consistent with the present teachings are particularly well-suited for operation at infrared wavelengths in the range of about 1.5 to about 20 microns, and may be configured as an antireflection coating, a high-reflection coating, a long-wavelength-pass filter, a short-wavelength-pass filter, a wide-bandpass filter, and a narrow-bandpass filter, among any other optical devices.
• Some embodiments in accordance with the present teachings provide an article comprising an optical-thin-film structure, wherein the optical-thin-film structure comprises alternating thin films of low-index material and high index material, the low-index material having a refractive index less than about 1.7, and the high-index material having a refractive index of at least about 1.7, wherein at least one of the thin films of low-index material consists essentially of a mixture of:
• (i) is suitable for reducing an average crystal size of the optical thin films of low- index material relative to an average crystal size of a low-index optical thin film consisting of 100 percent by weight of Barium Fluoride;
• Some other embodiments in accordance with the present teachings provide a method comprising forming a low-index optical thin film by co-evaporating Barium fluoride and a secondary fluoride compound on to a first surface; and forming a high-index optical thin film on the low-index optical thin film.
• FIG. 1A depicts an embodiment of an optical-thin-film structure in accordance with the illustrative embodiment of the invention.
• FIG. IB depicts an alternative embodiment of an optical-thin-film structure in accordance with the illustrative embodiment of the invention.
• FIG. 2A is a micrograph showing a conventional optical thin film of BaF 2 .
• FIG. 2B is a micrograph showing an optical thin film in accordance with the present teachings, wherein the optical thin film consists of BaF 2 and CaF 2 (95/5 weight percent, respectively).
• FIG. 2C is a micrograph showing an optical thin film in accordance with the present teachings, wherein the optical thin film consists of BaF 2 and AIF 3 (95/5 weight percent, respectively).
• FIG. 2D is a micrograph showing an optical thin film in accordance with the present teachings, wherein the optical thin film consists of BaF 2 and YF 3 (95/5 weight percent, respectively) and a layer of Germanium.
• FIG. 2E is a micrograph showing an optical thin film in accordance with the present teachings, wherein the optical thin film consists of BaF 2 and YbF 3 (95/5 weight percent, respectively).
• FIG. 2F is a micrograph showing an optical thin film in accordance with the present teachings, wherein the optical thin film consists of BaF 2 and YbF 3 (85/15 weight percent, respectively).
• FIG. 3 depicts an embodiment of an optical-thin-film structure in accordance with the present teachings, wherein the optical-thin-film structure is a Bragg mirror.
• FIG. 4A depicts an embodiment of an optical-thin-film structure in accordance with the present teachings, wherein the optical-thin-film structure is a first embodiment of an optical filter.
• FIG. 4B depicts an embodiment of an optical-thin-film structure in accordance with the present teachings, wherein the optical-thin-film structure is a second embodiment of an optical filter.
• FIG. 4C depicts an embodiment of an optical-thin-film structure in accordance with the present teachings, wherein the optical-thin-film structure is a third embodiment of an optical filter.
• FIG. 5 depicts an embodiment of an optical-thin-film structure in accordance with the present teachings.
• FIG. 6 depicts a flow diagram of a process for making an optical-thin-film structure in accordance with the present teachings.
• FIG. 1A depicts optical-thin-film structure 100 in accordance with the illustrative embodiment of the present invention.
• Structure 100 consists of layer 102 and layer 104.
• Layer 102 is a low-index optical thin film consisting essentially of a mixture of Barium Fluoride and one or more other "secondary" fluoride compounds.
• Layer 104 is a high-index optical thin film, which typically consists of one of Ge, Si, ZnS, or ZnSe.
• the refractive index of Barium fluoride at 10 microns is about 1.41, and is 1.352 at 14 microns.
• Barium fluoride is physically hard, and does not easily absorb moisture from the air compared to other fluoride compounds. These properties make Barium fluoride particularly attractive as a candidate for use as the low-index material in optical-thin-film structures for mid-IR applications.
• Thin-film deposition of Barium fluoride can be accomplished, for example, by thermal evaporation, electron-beam (E-beam) evaporation, or by using a sputter deposition process in which ions are accelerated onto a Barium fluoride target.
• thermal evaporation is used, fluoride deposition rates of over 2 nm/sec can be achieved, and the thin films of multi-layer mirrors can be deposited accurately if a quartz crystal oscillator monitor is used to measure the thickness of the films.
• the present inventors discovered that co-depositing (such as by co-evaporating) one or more secondary fluoride compounds with the Barium fluoride results in a smoother optical thin film.
• the co-deposition of the secondary fluoride compound reduces the average crystal size in the resulting optical thin film (relative to what it would have been had the optical thin film consisted of 100 percent Barium fluoride).
• the co-deposition can be performed without substantially compromising the refractive index or adhesion properties of the resulting thin film.
• YF 3 As to crystal shape, YF 3 , for example, has an orthorhombic Pnma space group that is different from that of Barium fluoride. And regarding lattice mismatch, the BaF 2 crystal has a lattice constant of 0.62 nm, whereas the CaF 2 crystal has a lattice constant of 0.54 nm. In fact, since all other fluorides compounds have a different crystal structure and/or lattice constant than BaF 2 , they are all suitable for causing defects in the crystal structure of BaF 2 and are expected to result in an optical thin film having a smoother surface than BaF 2 alone. Expectations aside, surface smoothness is readily determined by simple experimentation.
• the usefulness of such secondary fluoride compounds as co- deposition partners for Barium fluoride may ultimately be a function of suitability of the optical properties of such compounds for the optical application of interest (e.g., IR absorption losses in the wavelength region of interest, etc.)
• the secondary fluorides must possess adequate chemical stability. And of course, even if a particular fluoride compound is otherwise suitable based on the aforementioned characteristics, its cost, availability, and/or difficulty of synthesis may remove it from consideration.
• Optical thin films have been formed in which Barium fluoride was co-deposited via co-evaporation with (on an individual basis): YF 3 , YbF 3 , AIF 3 , CeF 3 , CaF 2 , and ThF 4 ,.
• the resulting optical thin films demonstrated significantly smoother interfaces than pure Barium fluoride films. The results of such experimentation are discussed below for CaF 2 , AIF 3 , YF 3 , and YbF 3 .
• FIGs. 2B through 2F are micrographs of respective low-index optical thin films 102B through 102F consisting of Barium fluoride and another fluoride compound, co- deposited in accordance with the present teachings.
• a thin layer of germanium (identified as layer 104B through 104F in the figures) was deposited on the surface of the low-index thin film to image surface roughness more clearly.
• Germanium was deposited via sputtering, but any of a variety of techniques known to those skilled in the art may suitably be used.
• Table I shows the composition of the low-index layer for FIGs. 2A through 2F.
• low-index optical thin film 102A consists solely of Barium fluoride, and serves as a comparative example.
• Layer 104A is germanium. Comparing the upper surface of layer 104A to the upper surface of layers 1O4B through 104E (FIGs. 2B-2E) demonstrates that low-index optical thin films 102B through 102E, which include five percent of co-deposited fluoride compounds, exhibit relatively less surface roughness than optical thin film 102A of pure Barium fluoride.
• FIGs. 2B through 2E demonstrate that the smoothness of the optical thin film is a function (among other parameters) of the particular fluoride compound that is co- deposited with Barium fluoride.
• co-deposited mixtures containing five weight percent of CaF 2 (FIG. 2B) or YbF 3 (FIG. 2E) result in a notable decrease in surface roughness.
• Co-deposited mixtures containing five weight percent of Yb 3 (FIG. 2D) result in even less surface roughness than CaF 2 or YbF 3 mixtures.
• AIF 3 (FIG. 2C) results in a greatly diminished surface roughness, showing the best performance at five weight percent of the secondary fluorides tested.
• the smoothness of the low-index optical thin film is also a function of the amount of the secondary fluoride compound(s) that is co-deposited with the Barium fluoride. This can be seen, for example, by comparing FIG. 2E (five weight percent YbF 3 ) with FIG. 2F (fifteen weight percent YbF 3 ), wherein the higher concentration of YbF 3 results in a smoother optical thin film.
• Temperature also plays a role in controlling the roughness of an optical thin film. Cooling of the deposited optical thin films (and the substrate upon which they are deposited) may reduce grain size, thereby improving the smoothness of the deposited optical thin films. Conversely, heating typically generates larger grains and more roughness.
• depositing "thin” optical thin films ( ⁇ 0.5 microns) on to substrates at room temperature it is typically not necessary to control temperature. However, depositing "thick" optical thin films (> 0.5 microns), such as for infrared mirrors and filters, increases the substrate temperature due to radiative heating from the hot evaporation source.
• the substrate temperature should be limited to no more than 100 °C. Preferably, the substrate temperature is maintained at less than 50 °C, and more preferably at 25 °C or less. Either form (active or passive) of temperature control is important reducing surface roughness as well as strain in an optical-thin-film structure.
• the concentration of the secondary fluoride can be increased. But because many of the secondary fluorides have higher IR absorption than Barium fluoride at wavelengths of interest, it is generally preferable to keep their concentration relatively low, such as 15 weight percent or less, notwithstanding the prospect of further improvements in the smoothness of the optical thin films at higher concentrations.
• secondary fluoride compounds suitable for use in conjunction with embodiments of the invention include those that: (1) reduce the average crystal size of a low-index optical thin film (relative to that of a low-index optical thin film consisting solely of Barium Fluoride), (2) have suitable optical properties for the particular optical application, and (3) have suitable chemical stability.
• Fluoride compounds that satisfy requirements (1) through (3) above for a variety of optical applications include, without limitation: CaF 2 , AIF 3 , YF 3 , YbF 3 , CeF 3 , ThF 4 , NaF, LiF, KF, SrF 2 , and MgF 2 .
• stress mitigation and interface adhesion may be improved via the following techniques.
• a surface treatment such as ion bombardment or the like is used to improve the adhesion between optical thin films of a multi-layer optical-thin- film structure.
• the surface is cleaned with Argon via ion beam, DC glow discharge, or RF glow discharge.
• Such bombardment cleans the surface of a deposited optical thin film to: (a) remove any contamination that might accumulate during the transition from one deposition step to the next, and (b) promote covalent bonding between the high-index material (e.g., Ge, Si, etc.) and the low-index fluoride-containing thin films.
• any adhesion-assisting layers may be reduced below 5 nanometers, or these layers may be completely eliminated. Ion bombardment has been shown to prevent delamination of the optical thin films and result in a more robust and durable optical-thin- film structure.
• a different surface treatment e.g., laser ablation, desorption, etc. is used to improve the adhesion between at least two of the optical thin films of a multi-layer optical-thin-film structure.
• FIGs. 3, 4A-4C, and 5 depict illustrative embodiments of optical-thin-film structures in accordance with the present teachings.
• the secondary fluoride is AIF 3 in an amount of 5 weight percent in the low-index optical thin film (BaF 2 at 95 weight percent), and the high-index optical-thin-films are Germanium. If present, the substrate is Silicon.
• one or more different secondary fluoride compounds may suitably be used, such as CaF 2 , YF 3 , YbF 3 , CeF 3 , ThF 4 ,, NaF, LiF, KF, SrF 2 , and MgF 2 , or any other fluoride compounds that satisfy the requirements previously discussed (/.e., reduce average crystal size of a Barium fluoride optical thin film, suitable optical properties, and chemical stability).
• the amount of such different secondary fluoride compounds in the low-index optical thin film may vary based on previously discussed considerations (e.g., absorption, etc.).
• a different material may be used as the substrate, such as glass, Ge, sapphire, or any other material commonly used as a substrate for optical devices. And/or a different material may be used as the high-index optical thin film, such as Silicon, Zinc Sulfide, or Zinc Selenide, etc.
• FIG. 3 depicts optical-thin-film structure 300 in accordance with an illustrative embodiment of the invention.
• Structure 300 is a partially reflective mirror (/.e., a Bragg mirror) for mid-IR applications.
• the partially reflective mirror has four optical thin films: 102 1 , 104 1 , 102 2 , and 104 2 .
• Optical thin films 102 1 and 102 2 are low-index optical thin films consisting of Barium fluoride and AIF 3 .
• Optical thin films 104 1 and 104 2 are high-index optical thin films consisting of Germanium.
• each optical thin film is , where is the wavelength of the IR and n is the refractive index of each particular optical thin film (commonly called "quarter-wave" layers or films). It is notable that the thicknesses depicted for the optical thin films are not to scale; the differences shown are for the purpose of readily distinguishing low- and high-index optical thin films from one another.
• Optical-thin-film structures 400A, 400B, and 400C in accordance with the present teachings function as mid-IR wavelength optical filters. These optical filters include two Bragg mirrors separated by an optically resonant cavity.
• FIG. 4A depicts structure 400A, which is capable of filtering wavelengths in the mid-IR range and passing a single wavelength
• Optical-thin-film structure 400A includes first Bragg mirror 408 1 A and second Bragg mirror 408 2 A separated by optically resonant cavity 410A.
• Bragg mirror 408 1 A includes five optical thin films: low-index optical thin films (102 1 , 102 2 , and 102 3 ) and high-index optical thin films (104 1 and 104 2 ), alternating as previously disclosed.
• Bragg mirror 408 2 A includes four optical thin films: low-index optical thin films (102 4 and 102 5 ) and high-index optical thin films (104 3 and 104 4 ), alternating as previously disclosed.
• the low-index optical thin films consist of a co-deposited mixture of Barium Fluoride and AIF 3 .
• the high-index optical thin films are Germanium.
• Optically resonant cavity 410A can be comprise any one of a number of high-index materials, such as fused silica, germanium, silicon, zinc selenide, yttrium aluminum garnet (YAG), etc.
• the optically resonant cavity can be other than a solid material, such as a liquid or gas, the latter of which may be under partial vacuum.
• FIG. 4B depicts structure 400B, which is capable of filtering wavelengths in the mid-IR range and passing plural wavelengths
• Optical-thin-film structure 400B comprises first Bragg mirror and second Bragg mirror 408 2 B, which are separated by optically resonant cavity 410B.
• Bragg mirror 408 1 B includes four optical thin films: low-index optical thin films (102 1 and 102 2 ) and high-index optical thin films (104 1 and 104 2 ), alternating as previously disclosed.
• Bragg mirror 408 2 B includes four optical thin films: low-index optical thin films (102 3 and 102 4 ) and high-index optical thin films (104 3 and 104 4 ), alternating as previously disclosed.
• the low-index optical thin films consist of a co-deposited mixture of Barium Fluoride and AIF 3 .
• the high-index optical thin films are Germanium.
• the length of cavity 410B varies in a transverse direction, wherein, in FIG. 4B, cavity 410B has a minimum length at the left side of optical filter 400B, and a maximum length CL max at the right side of the filter.
• CL max the wavelength of the light transmitted through filter 400B varies with transverse position.
• Optically resonant cavity 410B contains a gas, such as air, etc.
• FIG. 4C depicts optical-thin-film structure 400C, which, like structure 400B, is capable of filtering wavelengths in the mid-IR range and passing plural wavelengths to
• Bragg mirror includes five optical thin films similar to structure 400A: low-index optical thin films (102 1 , 102 2 , and 102 3 ) and high-index optical thin films (104 1 and 104 2 ).
• Bragg mirror includes four optical thin films similar to structure 400A: low-index optical thin films (102 4 and 102 5 ) and high-index optical thin films (104 3 and 104 4 ).
• the low-index optical thin films consist of a co-deposited mixture of Barium Fluoride and AIF 3 .
• the high-index optical thin films are Germanium.
• optical-thin-film structure 400B As in optical-thin-film structure 400B, the length of cavity 410C varies in a transverse direction, wherein, in FIG. 4C, cavity 410B has a minimum length at the left side of optical filter and a maximum length at the right side of the filter. As a consequence of this variation in cavity length, the wavelength of the light transmitted through structure 400C varies with transverse position.
• optically resonant cavity 410C of structure 400C comprises high-index material, such as the materials referenced for cavity 410A of FIG. 4A.
• respective optical-thin-film structures 300, 400A, 400B, and 400C do not include any adhesion-assisting layers at the interface between low- and high-index optical thin films.
• such optical-thin-film structures include such adhesion-assisting layers (e.g., see FIG. IB).
• FIG. 5 depicts optical assembly 500, which includes first optical-thin-film structure 508 1 deposited on first surface 514A of substrate 512, and second optical-thin- film structure 508 2 deposited on second side 514B of the substrate.
• First optical-thin-film structure 508 1 has four optical thin films: low-index optical thin films (102 1 and 102 2 ) and high-index optical thin films (104 1 and 104 2 ), alternating as previously disclosed.
• Second optical thin film structure 508 2 includes eight optical thin- films: low-index optical thin films (102 3 , 102 4 , 102 5 , and 102 6 ) and high-index optical thin films (104 3 , 104 4 , 104 5 , and 104 6 ), alternating as previously disclosed.
• both first and second optical-thin-film structures 508 1 and 508 2 include at least some low-index optical thin films consisting essentially of Barium fluoride and AIF 3 .
• the substrate is Silicon
• the high-index optical thin film is Germanium.
• FIG. 6 depicts a flow diagram of method 600 in accordance with an illustrative embodiment of the present invention.
• step S601 Barium fluoride and a secondary fluoride compound are co- evaporated onto a surface (e.g., a substrate, an existing optical thin film, an adhesion- assisting layer, etc.).
• step S602 (which, if conducted, is carried out during step S601), the substrate or optical thin film on to which the growing low-index optical thin film is being deposited is cooled, as is the growing optical thin film itself.
• the surface of the deposited low-index optical thin film is surface treated, such as by ion bombardment.
• step S604 an adhesion-assisting layer is deposited on the surface of the low-index optical thin film.
• step S605 a high-index material is deposited on the surface of the low-index optical thin film (or on the adhesion-assisting layer, if present).
• step S606 query if more optical thin films are to be deposited. If "yes,” then optionally perform steps S607 (ion bombardment) and S608 (deposit adhesion-assisting layer), and then loop back to step S601 to co-deposit a low-index optical thin film on to the just-deposited high-index optical thin film (or adhesion-assisting layer). As appropriate, in some alternative embodiments, the process can begin by depositing a high-index optical thin film, followed by the deposition of a low-index optical thin film.
• an optical-thin-film structure comprises at least one low-index optical thin film consisting essentially of Barium fluoride and at least one secondary fluoride compound that reduces the average crystal size of a low-index thin film (relative to that of a low-index optical thin film consisting solely of Barium Fluoride), and at least one high-index optical thin film.
• Embodiments of an optical-thin-film optical structure in accordance with the present invention may further comprise at least one of the following features, in any (non-conflicting) combination, among other features disclosed herein:
• the secondary fluoride compounds are selected from the group consisting of CaF 2 , AIF 3 , YF 3 , and YbF3.
• the secondary fluoride compounds are selected from the group consisting of CaF 2 , AIF 3 , YF 3 , YbF 3 , CeF 3 , ThF 4 , NaF, LiF, KF, SrF 2 , and MgF 2 .
• refractive index of the low-index optical thin film is less than about 1.7.
• the refractive index of the high-index optical thin film is at least about 1.7.
• the amount of Barium fluoride in the low-index optical thin film is in a range of about 75 to about 99 weight percent.
• the amount of Barium fluoride in the low-index optical thin film is in a range of about 85 to about 99 weight percent.
• the low-index optical thin film consists essentially of BaF 2 in an amount of 95 percent by weight and AIF 3 in an amount of 5 percent by weight.
• the low-index optical thin film consists essentially of BaF 2 in an amount of 99 percent by weight and AIF 3 in an amount of 1 percent by weight.
• the low-index optical thin film consists essentially of BaF 3 in an amount in an amount in a range of about 85 to about 95 percent by weight, and YbF 3 in an amount in a range of about 5 to about 15 percent by weight.
• the high-index optical thin film comprises a material selected from the group consisting of Ge, Si, ZnS, or ZnSe.
• optical-thin-film structure is selected from the group consisting of an antireflection coating, a high-reflection coating, a long-wavelength-pass filter, a short-wavelength-pass filter, a wide-bandpass filter, and a narrow-bandpass filter.
• optical-thin-film structure is physically adapted to reflect or filter infrared wavelengths in the range of about 1.5 to about 20 microns.
• the at least one low-index optical thin film and the at least one high- index optical thin film function as a partially reflective mirror.
• optical-thin-film structure comprises plural low-index optical thin films and plural high-index optical thin films, the low-index and high-index optical thin films alternating with one another and defining two partially reflective mirrors that are spaced apart from one another forming an optically resonant cavity in a region therebetween.
• optically resonant cavity comprises a low-index material.
• optically resonant cavity comprises a high-index material.
• optically resonant cavity comprises a gas
• optically resonant cavity comprises a gas under partial vacuum.
• an interfacial layer is disposed between at least some of the alternating optical thin films, the interfacial layer having a thickness in a range of about 0.1 to about 200 nanometers and comprising a material different than the low-index material and the high-index material.
• the interfacial layer comprises a material selected from the group consisting of ZnS, ZnSe, Hfo 2 , Y 2 O 3 , and AI 2 O 3 .
• a first optical-thin-film structure is disposed on a first surface of a substrate, and a second optical-thin-film structure is disposed on a second surface of the substrate, wherein at least one of optical-thin-film structures includes a low-index optical thin film consisting essentially of Barium fluoride and at least one secondary fluoride compound.
• the temperature is maintained at less than 100 °C.
• the temperature is maintained at less than 50 °C.
• the temperature is maintained at 25 °C or less.
• any numerical range recited herein is intended to include all sub-ranges encompassed therein.
• a range of "1 to 10" is intended to include all sub-ranges between (and including) the recited minimum value of about 1 and the recited maximum value of about 10, that is, having a minimum value equal to or greater than about 1 and a maximum value of equal to or less than about 10.

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